Online, robust, reagentless and low maintenance sensors that detect the presence and concentration of bacteria in fluids will have potential use in many areas where a continuous monitoring of bacteria can ensure microbial quality or decrease the number of manually collected samples and measurements. An example is related to monitoring microbial water quality in wells/water intake, water works and/or water distribution system.
Methods for analysis of particles in liquid samples are known from the prior art. In some of these methods, a plurality of images of the fluid is recorded and further analyzed in order to identify the characteristics of the particles.
Currently, the most advanced box-product online sensors and alarm systems for monitoring (microbial) water quality are turbidity sensors and particle counters. Turbidity, however, is only an indicative measurement, as it is not only bacteria that can change the turbidity of water. Furthermore, turbidity sensors lack sensitivity and have a relatively poor detection limit. On-line box-product particle counters for water quality measurements do have the sensitivity and detection limit required. They can, however, only detect particles with diameters down to 1-2 μm. Bacteria in water have sizes from 0.3 μm to 3 μm, with most of them having sizes below 1 μm. Thus, current particle counters are not effective for detecting bacteria in liquids and they cannot discriminate between bacteria and other particles. Furthermore, particle counters are prone to errors in the readout if there are deviations in the relatively large and constant flow (>50 ml/minute) through the measurement unit. The constant flow is often ensured by a large tubing system that utilizes gravity to ensure the constant flow.
Thus, the constant flow requirement is one of the main disadvantages of the prior art systems. Moreover, the online methods available on the market require huge amounts of water during the measurement period. In many applications (e.g., in a well), there are no drains available and thus the known systems cannot be used.